1. Field of the Invention
The invention relates to an apparatus for producing hydrogen radicals, and also to a cover for use with such an apparatus.
2. Description of the Related Art
A typical apparatus for producing hydrogen radicals by means of a hot filament is a hydrogen cracking cell (as reported in Japanese Journal of Applied Physics, Vol. 30, No. 3A, March, 1991, page L 402-L 404). FIG. 1 schematically illustrates a structure of the hydrogen cracking cell. The cell comprises a pipe 1 (composed of boron nitride (BN) and having an internal diameter of 4 mm), and a filament 2 (composed of tungsten (W), 100 mm long), disposed in pipe 1. First, an electrical current is run through filament 2 to heat it 2. Then, hydrogen gas 3 is fed through the pipe 1. Hydrogen gas 3 is dissociated into atomic hydrogens, (i.e.;) hydrogen radicals 4).
Japanese Unexamined Patent Public Disclosure No. 5-890 has suggests an apparatus for producing hydrogen radicals in which thermal capillaries are used in place of a hot filament. In this apparatus, hydrogen gas is fed through a plurality of capillaries composed of metal having a high melting point. The apparatus provides is claimed to provide a high production rate of hydrogen radicals.
Japanese Unexamined Patent Public Disclosure No. 61-59821 suggests a radical beam producing apparatus comprising (a) a discharge chamber, in which radicals are produced by means of high frequency discharge, using a magnetic field wherein plasma is gathered around an axis thereof, and (b) a plurality of orifices, for introducing the radicals (produced in the discharge chamber) into a process chamber in the form of beams. It is set forth that the apparatus provides in a layer forming process, a higher etching speed, without damaging a substrate.
Japanese Unexamined Patent Public Disclosure No. 1-232651 also suggests a radical beam producing apparatus. The apparatus comprises (a) a discharging device for receiving energy and for gas-discharging, (b) a plasma chamber for dissociating and exiting the gas discharged from the discharging device to thereby produce radicals, (the plasma chamber having an opening through which the thus produced radicals are drawn out), and (c) a magnet located adjacent to the opening. The Disclosure asserts that the apparatus provides radical beams free of ions and electrons.
The production of hydrogen radicals requires a very high temperature, specifically, in the range of 1500 to 2000 degrees centigrade. When hydrogen radicals are to be used for cleaning a substrate in a semiconductor manufacturing apparatus, there is concern that the substrate may be contaminated by gases leaked out of the apparatus that produces the hydrogen radicals. This concern is particularly significant when II-VI material, which has a high vapor pressure, is to be used in fabricating a semiconductor. Thus, it is absolutely necessary, in the production of hydrogen radicals to use a hot filament which generates a small quantity of heat relative to the filament temperature thereof. However, when a material having a high vapor pressure is used, the substrate might be contaminated by gases leaked due to heat derived from the filament.
A thinner filament generates a smaller quantity of heat relative to the temperature thereof, but provides a smaller cross-sectional area to the hydrogen gas, deteriorating the thereby production rate of hydrogen radicals. Conventionally when a hot filament is used it is in the pipe composed of boron nitride (BN) to avoid the deterioration of a production rate of hydrogen radicals. However, such arrangement is accompanied by gas with leakage from the BN pipe. Since the temperature of the filament ranges from 1500 to 2000 degrees centigrade, there is concern over boron (B) contamination.
This problem can be solved, for instance, by replacing the BN pipe with a pipe composed of a metal which has a high melting point. However, such replacement is accompanied by the problem of electrical insulation. If the pipe composed of metal having a high melting point comes in contact with a filament, there occurs a leakage of current, making it impossible to heat the filament fully up to the desired temperature. In order to avoid such a problem, the pipe diameter needs to be sufficiently large relative to the filament diameter, taking into consideration the successive change of the filament diameter over time. However, such a design is accompanied by the above-mentioned deterioration of the hydrogen radical production rate. This is a vicious cycle.
In addition, since hydrogen radicals are active, hydrogen radicals irradiated to an area other than a substrate react with material stuck to the internal surface of the hydrogen radical producing apparatus, thereby generating hydride and causing contamination. For instance, when hydrogen radicals were irradiated to a silicon substrate, in a semiconductor manufacturing apparatus having an internal surface to which material containing Te is stuck, it was observed by means of X-ray photo-electron spectroscopy (XPS) that a part of a surface of the Si substrate was contaminated with Te (as shown in FIG. 2).